A. Technical Field
The present invention relates to a water-absorbent resin and a production process therefor. More particularly, the present invention relates to a production process for a modified water-absorbent resin by carrying out a specific step, and a novel water-absorbent resin as surface-crosslinked with a polyhydric alcohol.
B. Background Art
In recent years, water-absorbent resins are widely used as one of materials constituted of sanitary materials, such as disposable diapers, sanitary napkins, and incontinent pads, for the purpose of causing the water-absorbent resins absorb much water. In addition to the sanitary materials, the water-absorbent resins are widely used as dripping sheets for soil water-holding agents and foods, for the purpose of absorbing or retaining water.
As to these water-absorbent resins, the following are known as their examples: hydrolyzed copolymers of starch-acrylonitrile (JP-B43395/1974), neutralized graft polymers of starch-acrylic acid (JP-A-125468/1976), saponified copolymers of vinyl acetate-acrylic acid ester (JP-A-14689/1977), and hydrolyzed copolymers of acrylonitrile or acrylamide (JP-B-15959/1978), or crosslinked polymers of these hydrolyzed copolymers, and partially-neutralized crosslinked poly(acrylic acid) (JP-A-84304/1980).
These water-absorbent resins are generally obtained by polymerizing and drying, and pulverizing and classifying when the occasion demands. However, the above water-absorbent resins are usually modified by further adding various compounds to the resultant water-absorbent resins so that the water-absorbent resins would have additional function after polymerizing and drying.
It is said that the above-mentioned water-absorbent resins should be excellent in the following properties: the absorption capacity, the absorption speed, the liquid permeability, the gel strength of hydrogel, the suction power to suck up water from a base material containing an aqueous liquid, and so on, upon contact with an aqueous liquid such as a body fluid. However, relations between these properties do not necessarily display positive correlations. For example, as the absorption capacity increases, some other properties such as liquid permeability, gel strength, and absorption speed deteriorate. In addition, there are some water-absorbent resins having higher absorption capacity, which form so-called fish eyes in contact with an aqueous liquid, and have extremely low absorption capacity under a load because water is not dispersed in the entirety of the water-absorbent resin particles.
When producing the water-absorbent resins, as to a method for modifying the above-mentioned water-absorption properties of the water-absorbent resin in good balance, namely, as to a method for modifying the water-absorbent resin, an art in which the neighborhood of the surface of the water-absorbent resin is crosslinked, what is called, a surface-crosslinking art is known. Examples of these crosslinking agents as used are polyhydric alcohols, polyglycidyl ethers, haloepoxy compounds, polyaldehydes, polyamines, and polyvalent metal salts.
The most important matter thought in this surface-crosslinking step is to surface-crosslink the surface of water-absorbent resin particles uniformly, and therefore, it is important that the water-absorbent resin before surface-crosslinking is uniformly blended with a surface-crosslinking agent. As to an art in which this water-absorbent resin before surface-crosslinking is uniformly blended with the surface-crosslinking agent, various methods are disclosed until now. For example, the following methods are known: a method which involves the use of crosslinking agents having a different solubility parameter together (JP-A-184320/1994 (corresponding to U.S. Pat. No. 5,422,405)); a method which involves the use of a specific material as the material of the inner surface of the blender, and involves add-blending an aqueous crosslinking agent liquid while being stirred in a high speed (JP-A-235378/1997 (corresponding to U.S. Pat. No. 6,071,976) and JP-A-349625/1999); and a method involves spraying a particulate liquid drop of a surface-crosslinking agent to bring a water-absorbent resin powder in a row state (JP-A-246403/1992).
As to methods for surface-crosslinking water-absorbent resins using these crosslinking agents, the following methods are known: a method which involves directly adding a crosslinking agent to a water-absorbent resin powder, or a composition obtained by dissolving a crosslinking agent in a small quantity of water or a hydrophilic organic solvent, and heat-treating if necessary (JP-A-180233/1983 (corresponding to U.S. Pat. No. 4,666,983), JP-A-189103/1984, and JP-A-16903/1986 (corresponding to U.S. Pat. No. 4,734,478)); a method which involves dispersing a water-absorbent resin in a mixed solvent of water and a hydrophilic organic solvent, and adding a crosslinking agent thereto to react with the water-absorbent resin (JP-B48521/1986); and a method which involves allowing a resin to react with a crosslinking agent in an inert solvent in the presence of water (JP-B-18690/1985 (corresponding to U.S. Pat. No. 45,418,771)).
Then, when a water-absorbent resin is surface-crosslinked, the moderate permeation of a crosslinking agent to the neighborhood of a water-absorbent resin powder is an important factor, and it is necessary that its process is favorable in view of industry.
In addition, the state of the water-absorbent resin is a powder in many cases. When the water-absorbent resin includes many particulate powders such that pass through a sieve having a mesh opening size of 150 μm, it may exercise a bad influence on the working environment due to causing dust, it may cause the blendability to decrease when blending with other substances, and it may cause formation of bridge in a hopper.
Until now, known examples of production processes for water-absorbent resins having a small amount of particulate powders include a method which involves adjusting the particle diameter by adjusting the extent of the polymerization or pulverization, or a method which involves classify-removing the particulate powders as caused. However, plenty of particulate powders (several to several tens percents) are caused in the production steps even if the above method is carried out. Therefore, the yield is greatly decreased when the particulate powders are classify-removed, and further abandoned. At the same time, there are disadvantages in view of abandoning cost.
Then, various methods for modifying a water-absorbent resin for the purpose of solving the above problems are proposed by granulating or recovering the particulate powders to granules by use of binders such as an aqueous liquid, wherein the particulate powders are inevitably caused in the steps of producing the water-absorbent resin (JP-A-101536/1986 (corresponding to U.S. Pat. No. 4,734,478) and JP-A-817200/1991 (corresponding to U.S. Pat. No. 5,369,148)). Preferred binders for the water-absorbent resin generally include water or an aqueous liquid in view of efficiency, safety, and production costs.
The steps of producing such a water-absorbent resin includes an modifying step by adding and blending a liquid material, such as adding a surface-crosslinking agent to the water-absorbent resin after polymerization and drying, or blending a binder to a water-absorbent resin including powders in order to reduce dust as caused. In addition, when carrying out an antimicrobial processing, an removal of odor, and besides, an modification of giving additional functions to a water-absorbent resin, which tend to increase in recent years, the water-absorbent resin is frequently blended with antimicrobial agents, deodorants, and besides, other additives, as a liquid material.
Furthermore, when the water-absorbent resin is surface-crosslinked or modified, a liquid material is added (preferably spray-added), and then the resultant mixture is heat-treated. However, even if the water-absorbent resin is heated at the same temperature (water-absorbent resin temperature or heat medium temperature), depending upon the kind of liquid materials, the improvement of the properties might be insufficient, and the properties might not be stabilized in a continuous production process.
When producing the water-absorbent resin, the step of blending the liquid material to the water-absorbent resin is essential so as to modify its various properties with good balance, and further, to give additional functions. However, the water-absorbent resin has a property of absorbing the liquid material rapidly when the water-absorbent resin comes into contact with the liquid material. Therefore, it is difficult to blend the liquid material with the water-absorbent resin uniformly.
In addition, the water-absorbent resin has a characteristic of increasing adhesion when the water-absorbent resin absorbs a liquid. Therefore, the water-absorbent resin excessively absorbing the liquid may be formed as an adhesive or piled material in a blending apparatus. When operating the blending apparatus in order to mass-produce the water-absorbent resin, the formation of such a piled material causes an overload for a driving motor of such as a driving shaft, and is a serious problem on operating the apparatus safely.
In addition, the surface-treatment, such as forming surface-crosslinking layers in a water-absorbent resin is tried so as to modify various properties of the water-absorbent resin in good balance in the above way. However, any treatment has above-mentioned problems, and there was no sufficiently satisfactory method in view of property and industry before.